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Appendix IL


tappets, so that the testing is continued without attention until
stopped by hand.

P. j8jr. Test Specimens.  Professor Carpenter has
experimented on specimens of various lengths, 2", 4", 6", and 8",
from which he deduces that the ultimate strength (as shewn by
highest point of stress-strain curve) is independent of specimen
length, and that percentage extension (ductility) at maximum load
is pretty uniform. The ductility at rupture is, however, variable,
being inversely as length of specimen up to 8" long, and after-
wards constant. This indicates the need of a standard length
of not less than 8"; but if maximum load and extension only are
required, length is of no importance.

P* 3#5-    Stress-Strain Diagrams.

Mechanical Hysteresis.If tension experiments are made upon

rods or beams well within the elastic limit, by means of loads
gradually increased from zero to a maximum and then gradually
decreased to zero again, the ^ascending stress-strain curve will not
agree with the descending one. The first will follow the true
elastic line, but the second will slope more steeply and meet the
strain base at a point somewhat to the right of the origin. The
apparent permanent strain, called lag, will gradually disappear,
however. The area enclosed by the two curves has been called
mechanical hysteresis (from its similarity to magnetic hysteresis)